1. Field of the Invention
The present invention relates to a directional beam antenna device and a directional beam controlling apparatus, and more particularly to a directional beam antenna device and a directional beam controlling apparatus which are suitable as an antenna for a mobile earth station in communications using a satellite.
2. Description of the Related Art
In a case where an earth station is a mobile object such as a vehicle or a vessel in communications using a satellite, an antenna for the earth station is required to be capable of effecting communication satisfactorily without being affected by a change in the location or attitude of the mobile object. In a case where the moving range of the mobile object is relatively narrow, even if the location of the mobile object has changed, the change in the angle of the satellite as viewed from the mobile object in a vertical plane including both the mobile object and the satellite (hereafter, this angle in the vertical plane will be referred to as the "elevation angle," and the direction in the vertical plane as the "direction of the elevation angle") is small. Therefore, an antenna apparatus is used which is arranged such that a low-gain antenna which is omnidirectional in a horizontal plane is fixedly disposed such that the direction in which the antenna gain becomes maximum in the vertical plane will substantially coincide with the elevation angle of the satellite.
However, to obtain more satisfactory communication quality, by using an antenna having a high gain and sharp directivity, i.e., a directional antenna having a directivity pattern in which the half-power beam width of the main lobe of an antenna beam is small, it is desirable to change and control the direction of the antenna beam (the direction of the boresight (the principal axis of directivity) of the antenna) in correspondence with the direction of the satellite which changes with the change in the location or attitude of the mobile object.
As an antenna apparatus for a mobile earth station which is capable of changing the direction of the antenna beam, an apparatus is known in which, as shown in Japanese Patent Application Laid-Open (JP-A) No. 6-283919, a circular plane antenna is rotatably supported at two opposing points of its end portions, and the plane antenna is rotated about an axis connecting the two points so as to control the elevation angle of the antenna beam, and the overall antenna (the plane antenna and a member supporting the plane antenna) is rotated about an axis extending in the vertical direction so as to control the direction in the horizontal plane of the antenna beam (hereafter, the angle extending along the horizontal direction will be referred to as the "azimuth angle," and the direction in the horizontal plane as the "direction of the azimuth angle).
However, with the antenna device having the above-described configuration, the change in the rotation angle, per se, at a time when the plane antenna is rotated about the axis connecting the two points supporting the plane antenna becomes the change in the elevation angle of the antenna beam, so that there has been a problem that a complex elevation-angle controlling mechanism is required for accurately controlling the elevation angle of the antenna beam. In addition, in the case where the above-described antenna device is used by being mounted on a mobile object such as a vehicle, the antenna itself vibrates in the direction of the elevation angle due to the vibration of the mobile object, and the elevation angle of the antenna beam is frequently offset from the direction of the satellite, so that it has been difficult to put such an antenna device to practical use.
In addition, Japanese Patent Application Laid-Open (JP-A) No. 8-162833 discloses a vehicle-mounted antenna device for satellite communication which is arranged such that an antenna supporting member is supported on a base in such a manner as to be rotatable about a first rotational axis extending along a vertical direction, a plane antenna is supported on the antenna supporting member in such a manner as to be rotatable about a second rotational axis which is not perpendicular to the first rotational axis and is inclined by an angle substantially equal to the elevation angle of a satellite, and a driving mechanism for rotating the antenna supporting member about the first rotational axis is provided.
With the plane antenna in the above-described antenna device, the direction of the antenna beam coincides with an axial direction perpendicular to the aperture of the antenna, i.e., coincides with the direction of the second rotational axis. In addition, with the above-described antenna device, only the rotation of the antenna supporting member about the first rotational axis is actively controlled in correspondence with a change in the location or direction of the mobile object. The plane antenna is passively rotated about the second rotational axis by the torsional force occurring due to the rotation of the antenna supporting member, and the occurrence of twisting of a cable and the like accompanying the relative rotation of the plane antenna with respect to the base is prevented.
However, with the above-described antenna device, the angle of the plane antenna in the vertical plane is fixed to an angle perpendicular to the second rotational axis, and the elevation angle of the antenna beam of the plane antenna is also fixed, it is impossible to control the elevation angle of the antenna beam in a case where the elevation angle of the satellite as viewed from the mobile object has changed by accompanying a change in the attitude in the direction of the elevation angle of the mobile object (e.g., in a case where a vehicle as the mobile object is traveling on a road having a gradient) or in a case where the moving range of the mobile object is so wide that the change in the elevation angle of the satellite cannot be ignored. Thus there has been a problem in that the communication quality in satellite communication deteriorates.